Method of restricting chip movement upon bonding to rigid substrate using spray coatable adhesive

ABSTRACT

A method of bonding a chip to a wafer at precise alignment suitable for fabricating a heater chip in an ink jet printhead is provided. The method includes spray coating an adhesive composition on a surface of a substrate, aligning and tacking at least one chip to the substrate coated with the adhesive composition, exposing the substrate tacked with at least one chip coated with the adhesive composition to radiation and heat, and performing thermal compression bonding. The method uses a spray coatable adhesive composition comprising a thermally activated adhesive and a photoacid generator.

CROSS REFERENCES TO RELATED APPLICATIONS

This patent application is related to and claims priority to U.S. Provisional Patent Application Ser. No. 61/358,179, filed Jun. 24, 2010, entitled “Formula and Process for Ejection Chips in Micro-fluid Applications” and assigned to the assignee of the present application.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

REFERENCE TO SEQUENTIAL LISTING, ETC.

None.

BACKGROUND

1. Field of the Invention

The present disclosure generally relates to methods of fabricating micro-fluid ejection devices, such as printheads used on inkjet printers. More particularly, the present disclosure relates to a method of fabricating a heater chip to be used in a printhead. The disclosed method uses a spray coatable adhesive which restricts the movement of the heater chip when it is bonded to a wafer.

2. Description of the Related Art

In the field of micro-fluid ejection devices, ink jet printers are an example application where miniaturization continues to be pursued. This includes miniaturization of ejection chips or dies to reduce material cost of ejection heads. In reducing the ejection chip size, it is usually the chip width that must be decreased while the chip length would be tailored to the cost/performance requirements of a particular printing application. These ejection chips are typically coupled with a rigid support substrate such as a silicon substrate having one or more flow features. So as ejection chips get smaller, the bonding and sealing widths to the support this rigid silicon substrate must be reduced to much less than 500 microns. This miniaturization of ejection chips then leads to challenges relating to techniques and adhesives used for bonding the chips on the support substrate. This bonding must provide narrow and robust seals and proper chip alignment to flow features on the support substrate.

When using a thermally activated adhesive to bond miniaturized chips or dies onto a wafer substrate, a problem arises with respect to the alignment of the chips or dies to the wafer. The chips or dies move during the thermal compression bonding process. This results in the misalignment of the flow features on the chips or dies to the flow features of the wafer substrate. Unfortunately, the measured die or chip movement is more than 100 microns. This amount of movement is unacceptable because it is beyond the alignment tolerance of the chip to wafer when used for a heater chip in a printhead. Thus, there is a need for an improved bonding technique to be used when manufacturing miniaturized ejection head components in a printhead in an ink jet printers.

SUMMARY

The present disclosure provides a method of bonding a chip to a wafer at precise placement or alignment. The method includes providing a substrate, coating the substrate with spray coatable adhesive composition comprising thermally activated adhesive and photoacid generator, aligning and tacking at least one chip to the adhesive coated substrate, exposing to radiation and heat, and performing thermal compression bonding. This method decreases chip movement to less than 100 microns during thermal compression bonding.

The features and advantages of the present disclosure will be more understood through the detailed description and in reference to the figures which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional diagram of a heater chip fabricated in accordance with the bonding method of the present disclosure.

FIG. 2 is a schematic diagram of a chip to wafer bonding process according to the present disclosure.

FIG. 3 is a graphical illustration of the average die movement measured after thermal compression bonding of silicon die on wafer using the bonding method of the present disclosure.

FIG. 4 is a graphical illustration of the adhesive strength in an ink environment of the adhesive composition used in bonding a chip to a wafer according to the method of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but these are intended to to cover the application or implementation without departing from the spirit or scope of the claims of the present disclosure. It is to be understood that the present disclosure is not limited in its application to the heater chip of an ink jet printer set forth in the following description. The present disclosure is capable of other embodiments and of being used in various applications. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Further, the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

With reference to FIG. 1, there is shown, in partial cross-sectional view, a portion of a heater chip 10 for an ink jet printhead being fabricated in accordance with a bonding method of the present disclosure. The heater chip 10 includes a support substrate 20, a silicon die 40, and a spray coatable bond adhesive 30 formed and located between the support substrate 20 and the die 40 so as to attach them to one another. The silicon die and the support substrate include vias 50 as ink flow features. The bond adhesive 30 includes a thermally activated adhesive and a photoacid generator.

The above-described heater chip 10 is fabricated in accordance with the bonding method of the present disclosure as illustrated in the flow diagram of FIG. 2. The bonding method includes spray coating an adhesive composition on a cleaned or treated wafer surface, aligning and tacking dies on the adhesive coated wafer, exposing the wafer coated with adhesive composition to radiation and heat, performing thermal compression bonding, and baking for final cure of the adhesive.

The wafer can be a silicon wafer and may include one or more vias and alignment fiducials (reference points). The one or more vias may be formed through deep reactive ion etching (DRIE) process. The wafer may be cleaned with a combination of oxygen and argon plasma. The cleaned wafer surface may be further treated with silane adhesion promoter. The silane adhesion promoter may be applied with a spinner followed by a 25 minute oven bake at 112° C.

The spray coatable adhesive composition may be applied onto the wafer surface using commercially available spray coating equipment such as the spray coating equipment available from the EV Group of Phoenix, Ariz. under the trade name EVG-101. The spray coated adhesive is preferably baked at 70° C. for 40 minutes in a convection oven to remove the solvent.

The die to be bonded on the wafer may be a silicon die such as the silicon die of the heater chip for the ink jet printer. The silicon die may include a photo imageable nozzle plate (PINP). So it is preferable to apply an adhesive composition than can be cured at a temperature below the glass transition temperature of the PINP, preferably at temperature below 140° C. to prevent damage of PINP during the bonding process.

The alignment and tacking conditions are selected such that the die surface is sufficiently wetted by the adhesive coating with undue squeeze out and inadvertent cure of the adhesive coating. The total time and temperature for aligning and tacking the dies are held within a range in which the adhesive coating remains tacky with minimal initiation of cross-linking. The dies are preferably aligned at the wafer surface heated at about 70° C. to about 80° C. and tacked at pressures of about 1 to about 5 psi.

Exposing the wafer with the tacked dies to radiation and heat solidifies the exposed area of the adhesive coating and restricts the movement of the die upon thermal compression bonding. The wafer with the tacked dies is preferably exposed to an ultraviolet (UV) light of 364 nm at 1 J/cm² and then baked at 70° C. for 1 minute. Thermal compression bonding is then performed and followed by a final curing step to complete the bonding process. The final curing may be conducted at a temperature of about 130° C. for about 2 hours. After the final cure, the bonded die-to-wafer is diced into die-to-support substrate assembly preferably for used as heater chip of ink jet printer.

The spray coatable adhesive composition used in bonding the chips to the wafer according to the method of the present disclosure is prepared to include a photoacid generator and a thermally activated adhesive. The thermally activated adhesive comprises an epoxy based resin and thermal acid generator. The adhesive composition is also prepared to have a low viscosity which enables it to be spray coatable allowing for ease of application. It is preferable that the adhesive composition has a viscosity of less than 10 cps, The adhesive composition is prepared to comprise an epoxy based resin, a thermal acid generator, a to photoacid generator and a solvent.

The epoxy based resin includes a mixture of a solid epoxy resin and a liquid or semi-liquid epoxy resin. A uitable example of the solid epoxy resin includes a bisphenol A epoxy resin available from Hexion Performance Chemicals, Inc. under the trade name EPON 1007F having an epoxide equivalent of greater than about 1000 g/eq. An “epoxide equivalent” is the number of grams of resin containing 1 gram-equivalent of epoxide. An example of liquid or semi-liquid epoxy resin includes a hydrogenated bisphenol A epoxy resin available from CVC Thermoset Specialties Division of Emerald Performance Materials under the trade name EPALLOY 5000 having an epoxide equivalent of about 210 to about 230 g/eq. EPON 1007F and EPALLOY 5000 are difunctional epoxy resins and a person of ordinary skill in the art can calculate its average molecular weight based on the epoxide equivalent. EPON 1007F is a high molecular weight epoxy resin having an average molecular weight of greater than 2000 Daltons. EPALLOY 5000 is a low molecular weight epoxy resin having an average molecular weight of less than 500 Daltons. The ratio of the solid epoxy resin to the liquid or semi-liquid epoxy resin is controlled to provide an adhesive composition being non-tacky at ambient condition. This allows for ease of handling and transport while providing tack when exposed to heat for ease of tacking the dies. The ratio of the solid epoxy resin to liquid or semi-liquid epoxy resin in the adhesive composition may be in a range from about 80:20 to about 65:35. Most preferably the ratio is 65:35.

A suitable example of thermal acid generator is antimony-based thermal acid generator such as ammonium hexafluoroantimonate thermal acid generator available from King Industries under the trade name NACURE XC-7231 having an activation temperature of about 90° C. The concentration of thermal acid generator in the adhesive composition may be in an amount ranging from about 1.5 to about 3 parts by weight per 100 parts by weight of the epoxy based resin. A higher concentration of thermal acid generator results in ink flocculation issues while lower concentration results in insufficient curing. The thermal acid generator may preferably be in an amount of about 1.5 to about 2 parts by weight per 100 parts by weight of the epoxy based resin. The most preferred amount of thermal acid generator is in an amount of 1.5 parts by weight per 100 parts by weight of the epoxy based resin. This preferred amount of thermal acid generator results in adhesive composition having a glass transition temperature of about 90° C. to about 100° C., and provides sufficient curing, good ink compatibility and good adhesion.

The photoacid generator allows the rheology of the adhesive composition to change upon exposure to radiation and heat. A suitable example of a photoacid generator is a tris(trifluoromethanesulfonyl)methide photoacid generator available from Ciba Inc. under trade designation GSID 26-1. The photoacid generator may preferably be in an amount of about 1.5 to about 2 parts by weight per 100 parts by weight of the epoxy based resin. The most preferred amount of photoacid generator is in an amount of 1.5 parts by weight per 100 parts by weight of the epoxy based resin. The spray coatable adhesive composition may preferably include the thermal acid generator and the photoacid generator at a weight ratio of about 1:2 to about 2:1. The most preferable weight ratio of the thermal acid generator to the photoacid generator is 1:1.

A suitable example of solvent to provide appropriate viscosity of the adhesive composition is methyl ethyl ketone (MEK) or propylene glycol monomethyl ether acetate (PGMEA). The adhesive composition may have a viscosity of less than about 10 cps. More preferably, the adhesive composition may have a viscosity of about 1 to about 2 cps making it suitable for spray coating. MEK is a volatile solvent and has a lower boiling point than PGMEA. The adhesive composition most preferably includes a MEK solvent which provides good spray coating performance and sufficient solvent removal.

The adhesive composition optionally includes a silane adhesion promoter. Suitable example of silane adhesion promoter is an epoxy silane such as 3-glycidyloxypropyl trimethoxysilane commercially available from Sigma Aldrich Corporation. Adhesion may also be improved by applying a silane adhesion promoter to at least one of the adhering surfaces. These adhering surfaces may include the adhering substrate surface, die surface or adhesive coating surface.

The photoacid generator in the adhesive composition helps in keeping the die in place during thermal compression bonding and final curing. During UV radiation exposure, the photoacid generator in the adhesive composition is activated and forms super acid which initiates epoxy cross-linking reaction upon additional heat exposure. The exposed adhesive area cross-links and solidifies which then prevents die movement in subsequent thermal compression bonding and final curing. FIG. 3 illustrates average displacement of silicon dies from corresponding alignment fiducials in silicon wafer after thermal compression bonding using adhesive formulations listed in Table 1. These average die displacements are measured along X and Y axes.

TABLE 1 Thermal Acid Photoacid Maximum EPON1007F/ Generator Generator Die Adhesive EPALLOY5000 per hundred per hundred Movement Formulation resin weight ratio resin (phr) resin (phr) (μm) 1 65/35 3 0 160 2 65/35 1.5 1 40 3 65/35 1.5 1.5 6 4 65/35 2 1.5 15 5 65/35 3 1.5 10 6 65/35 1.5 3 6

With reference to FIG. 3, an average die movement of more than 100 μm was measured along X axis when using adhesive composition without photoacid generator, such as the Adhesive Formulation 1. Average die movement of less than 100 microns was measured along both X and Y axes when bonding using adhesive compositions containing photoacid generator, such as Adhesive Formulations 2, 3, 4, 5 and 6. The use of adhesive composition containing photoacid generator in bonding the silicon dies to silicon wafer can restrict die movement to less than 10 microns as observed with Adhesive Formulations 3 and 6.

The adhesion performance of the adhesive formulations listed in Table 1 was evaluated. In reference to Table 2, Adhesive Formulation 1, not containing photoacid generator, provides better adhesion even without additional silane treatment on the adhesive coating surface during bonding. With Adhesive Formulation 1, a force of above 200 N was required to peel the bonded silicon die from the wafer after soaking in water at a temperature of 100° C. for 8 hours. On the other hand, a peeling force of less than 200 N was required when using Adhesive Formulations 3, 4, 5 and 6, containing photoacid generator, without additional silane treatment on the adhesive coating surface during bonding. But, adhesion strength of the adhesive composition used in the bonding method of the present disclosure to can be improved by an additional silane treatment on the adhesive coating surface during bonding, as shown by the results of Adhesive Formulation 3 in Table 2 in which peeling force increased from 145 N to 225 N.

TABLE 2 Adhesive Formulation 1 2 3 4 5 6 Adhesion Strength 225 205 145 158 150 110 in terms of Peel Force, N (w/o silane treatment on top adhesive coating surface) Adhesion Strength in 220 225 terms of Peel Force, N (w/silane treatment on top adhesive coating surface) Minimum Seal Width, 80 80 μm (through Helium Leak test) Corrosion Resistance good Ink Flocculation Pass Pass Pass Robustness Ink Resistance, wt. % −0.15 0.60 1.22 change (dye based ink) Ink Resistance, wt. % −1.00 0.39 0.79 change (pigment based ink)

FIG. 4 graphically illustrates the average force required to peel a silicon die bonded on a wafer using Adhesive Formulation 492-34 with additional silane treatment on adhering surfaces, and taken before (T0) and after one week (T1) soaking in cyan ink (Dye-C), magenta pigment ink (Pigment-M) and mono pigment (Mono) at a temperature of 60° C. There was no degradation on the adhesive strengths of the Adhesive Formulation 3 after soaking in ink at a temperature of 60° C. for 1 week. This confirms that the adhesive composition used for bonding chips to wafer according to the method of the present disclosure, maintains good adhesion performance with an ink environment. It can also to provide a robust ink seal having good corrosion and ink resistance.

As presented in Table 2, the bond formed from the Adhesive Formulation 3 showed a weight change of 1.22% after soaking in dye based ink at a temperature of 60° C. for 10 weeks, and a weight change of 0.79% after soaking in pigment based ink at a temperature of 60° C. for 10 weeks. It also passed ink flocculation robustness test in which there was no significant growth of pigment ink particles after soaking in ink at a temperature of 60° C. for 4 weeks. This indicates that the adhesive composition used in the bonding method of the present disclosure can provide a robust ink seal having good resistance to ink and corrosion.

The adhesive composition being prepared for the bonding method of the present disclosure can reliably seal at a width as narrow as 80 μm, as confirmed by the helium (He) leak test result of Adhesive Formulation 3 presented in Table 2. The basic leak test structure consists of a silicon or glass wafer with fluidic ports bonded to another wafer consisting of etched channels. The etched channels are paired through a seal having widths ranging from 80 μm to 400 μm. The wafer is positioned on an X/Y table such that the pressurized He flow channel is over one access port, and an adjacent “sniff/sense” channel being separated from the He flow channel with the seal at a width of interest is over another access port leading to a mass spectrometer (MS). The seals are lowered onto the test wafer. The tester evacuates both channels and then opens the valve which supplies He at about 1 atm (14.7 psia) pressure to the He flow channel. The MS detects the He that leaks through the seal which separates the He flow channel from the sniff/sense channel. The MS reads over time the He flow rate coming from the “sniff/sense” access port. The tester waits for the leaked He flow rate to stabilize, i.e. around 30 seconds. A leaked He flow rate of less than 1×10⁻8 cm³/s is considered as “pass”.

An example preparation of the spray coatable adhesive composition used for bonding chips to wafer according to the method of the present disclosure is presented in Table 3. It is to be understood that this adhesive composition, its corresponding evaluation results and bonding method that have been described provide an example illustration of the present disclosure and should not be construed as a limitation of the present disclosure.

TABLE 3 Trade Name/ Component Common Name Chemical Description Loading Supplier Resin EPON 1007F solid bisphenol A epoxy resin 65 wt. % resin Hexion Performance Chemicals EPALLOY 5000 viscous liquid hydrogenated 35 wt. % resin CVC Thermoset bisphenol A epoxy resin Specialties Thermal Acid NACURE XC-7231* ammonium   1.5 phr King Industries Generator hexafluoroantimonate Photoacid GSID 26-1 tris(trifluoromethanesulfonyl)methide   1.5 phr Ciba Inc. Generator Adhesion Silane 3-glycidyloxypropyl  1.35 phr Sigma Aldrich Corp. Promoter trimethoxysilane Solvent MEK methyl ethyl ketone 1560 phr Sigma Aldrich Corp. *NACURE XC-7231 is pre-disolved in gama-butyrolactone (Sigma-Aldrich) at weight ratio of 50:50

The foregoing description of several methods and an example of the present disclosure has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise steps and/or forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto. 

1. A method of bonding a chip to a wafer, comprising: providing a substrate; spray coating an adhesive composition on a surface of the substrate, the adhesive composition including an epoxy based resin, a thermal acid generator, a photoacid generator and a solvent; aligning and tacking at least one chip to the substrate coated with the adhesive composition; exposing the substrate tacked with at least one chip coated with the adhesive composition to radiation and heat; and performing thermal compression bonding.
 2. The method of claim 1, wherein the providing of substrate comprises providing a silicon wafer having vias. 